Heteroplasmy of chloroplast DNA in Medicago

Two chloroplast DNA (cpDNA) regions exhibiting a high frequency of intra- or inter-species variation were identified in 12 accessions of the genus Medicago. Restriction maps of both regions were prepared for alfalfa, and the probable nature of the events causing the DNA differences was identified. Specific DNA fragments were then cloned for use in identification of variants in each region. Two each of M. sativa ssp. varia and ssp. caerulea and one of six M. sativa ssp. sativa single plants examined possessed cpDNA heterogeneity as identified by screening extracts for fragments generated by the presence and absence of a specific Xba I restriction site. Three plants of M. sativa ssp. sativa, two of each of sspp. varia and caerulea, and three M. scutellata were also examined for single-plant cpDNA heterogeneity at a hypervariable region where differences resulted from small insertion-deletion events. A single M. scutellata plant with mixed cpDNAs was identified. Sorting out was seen when one spp. sativa plant with mixed plastid types identifiable by the Xba I restriction site difference was vegetatively propagated. This indicated that the initial stock plant was heteroplastidic. Controlled crosses will be required in order to test whether heteroplasmy results from chloroplast transmission in the pollen and to examine the dynamic of sorting out. However, heteroplasmy is apparently not a rare situation in Medicago.